For MC build 1, Spellthief is from 3.5's Complete Adventurer, Witch is from PF1's Advanced Player's Guide (
class here,
spell list here), and Daggerspell Mage is also from Complete Adventurer. The build also uses the potent Master Spellthief feat from 3.5's Complete Scoundrel, basically giving you full Steal Spell progression (though not the other class features) and full Witch caster level (though not the slots of a single-classed Witch).
For MC build 2,
Unchained Monk is from Pathfinder Unchained and is their second attempt at fixing Monk,
Arcanist is from PF1's Advanced Class Guide and uses the Sorcerer/Wizard spell list, and Enlightened Fist is from 3.5's Complete Arcane. Also from Complete Arcane is the Practiced Spellcaster feat that helps you only fall behind linearly rather than quadraticly. Note that I'm giving Arcanist fewer Exploits from levels and the equivalent of a d4 Hit Die, however; it's clearly "tier 1" territory, and thus doesn't need the extra help.
For MC build 3, Swordsage is from 3.5's Tome of Battle: the Book of Nine Swords (what, did you expect me
not to use martial initiators at this power level?),
Wizard is as seen in 3.5 because as mentioned "tier 1" casters don't need the extra help, and
Bladecaster is from Dreamscarred Press's Path of War — its copyright-dodging answer to ToB:Bo9S. Note that I am letting Swordsage take the
Riven Hourglass discipline from Dreamscarred's second major book for the subsystem, Path of War Expanded; since Swordsage can already light a fire against the air with the Desert Wind discipline and teleport and throw shadows with the Shadow Hand discipline, I don't see why they shouldn't also be able to cut time with Riven Hourglass. (Basically Swordsage is the class that actually lives up to ToB:Bo9S's reputation of being "too anime" and "just giving fighters magic", so I'm leaning into that here.) This build also uses Practiced Spellcaster.
For MC build 4,
Warder is from Path of War and the
Zweihander Sentinel archetype is from the same, Erudite has two different versions but I'm using the take from Dragon Magazine #319 because the version in the back of Complete Psionic got hit too hard with the nerf bat even for my tastes, and
Awakened Blade is from Path of War (though Path of War Expanded modifies it to also grant access to the
Sleeping Goddess discipline on top of any you already had access to). Erudite uses the same power list as Psion and Wilder; you can see most of Dreamscarred Press's version of the list
here, and see most of WotC's version of the list in 3.5's Complete Psionic, though there are scattered books for both with other powers. Note that I do intend to give the Erudite access to talents — 0-level psionic powers — as per Dreamscarred's take on psionics. Much as the previous two builds use Practiced Spellcaster, this one uses Practiced Manifester from Complete Psionic.